Technical Q&A QA1297

Don't forget to cancel your Bonjour resolve

Q: After I Resolve a Bonjour service in order to get its IP addresses, do I need to cancel the Resolve operation?

A: Yes. Leaving the Resolve operation running places an unnecessary burden on the network because you end up sending query packets every couple of minutes in order to keep the DNS resource records up-to-date in the local machine's Bonjour cache. You should only Resolve the service at the moment you need to connect to it, and you should cancel the Resolve immediately after successfully connecting to the service. Manually stopping the Resolve also holds for any resolution involving a timeout scheme. Starting in Mac OS X 10.3, failure to cancel the Resolve will cause the following message to appear in the system log (/var/log/system.log).

Listing 1: system.log message

Oct 11 12:57:54 admin mDNSResponder[266]: 37755: DNSServiceResolver(Ice Cube._http._tcp.local.) has remained
active for over two minutes. This places considerable burden on the network.

The process by which you cancel the Resolve operation(s) depends on the method you are using.

• If you are using DNSServiceResolve, you need to call DNSServiceRefDeallocate.

• If you are using CFNetServiceResolve or CFNetServiceResolveWithTimeout, you need to make the following calls:

  Listing 2: CFNetServiceResolve Cancellation

  CFNetServiceSetClient(netService, NULL, NULL);
  CFNetServiceUnscheduleFromRunLoop(netService, runLoop, runLoopMode);
  CFNetServiceCancel(netService);
  CFRelease(netService);
  

• If you are using NSNetService, you need to make the following calls:

  Listing 3: NSNetService Cancellation

  [NSNetService stop];
  [NSNetService release];
  

  Or you can use [NSNetService resolveWithTimeout:] and specify a timeout interval. An example of the calls needed would look similar to the following:

  Listing 4: NSNetService Timeout Example

  if ([someNetServiceInstance respondsToSelector:@selector(resolveWithTimeout:)]) {
       // Mac OS X 10.4 "Tiger" or later
       [someNetServiceInstance resolveWithTimeout:20];
  } else {
       // Mac OS X 10.3 "Panther" or earlier
       [someNetServiceInstance resolve];
       NSTimer * stopTimer = [NSTimer timerWithTimeInterval:20
                                      target:someNetServiceInstance
                                      selector:@selector(stop)
                                      userInfo:nil
                                      repeats:NO];
       [[NSRunLoop currentRunLoop] addTimer:stopTimer forMode:NSDefaultRunLoopMode];
  }
  

  IMPORTANT: As mentioned above, be sure to stop your Resolve once you've succesfully connected to a service even when you are using a timeout scheme. Otherwise CF/NSNetServices will continually call you back with newly discovered IP addresses until the timeout value has been reached.

  Note: For Mac OS X 10.4 [NSNetService resolve] times out automatically after 5 seconds.

There are some rare applications that need to keep a Resolve running in order to monitor for TXT record changes. iChat, for example, continuously monitors for changes to a buddy's status message, which is stored in the Bonjour TXT record. If your application requires this type of functionality, starting in Mac OS X 10.4 you can now monitor for TXT records using CFNetServiceMonitor and/or [NSNetService startMonitoring]. If you want your application to support Mac OS X 10.3 use the DNSServiceDiscovery APIs, located at /usr/include/dns_sd.h.

Document Revision History

Posted: 2007-08-30